Commercial and theatrical lighting systems, for example, make extensive use special lamps, such as lamps known commercially as PAR36 and AR111. These are lamps made to different specifications, designed to be used interchangeably in low voltage spotlights, for example, used in track lighting, specialized theatrical and commercial lighting and the like. These standardized lamp types are provided at the back of their respective lamp bodies with specially configured, strip-like contact elements, by means of which the lamps are connected to a suitable power source. 
It is desirable to design and construct spotlight housings to accommodate different types of lamps, such as the PAR36 and AR111, in the interest of user convenience and manufacturing economies. Additionally, a user of the spotlight units may choose to change the type of lamp mounted therein from time to time, in order to achieve different lighting effects available from the different lamps. 
Because of a fundamentally different physical character of the contact structure at the back of the PAR36 and AR111 lamps, for example, it has been common practice to design spotlight units utilizing such lamps to provide for mounting the lamps by physically engaging their outer peripheries. In this respect,  the lamps are of similar diameters and are provided with similar external flange configurations, enabling seating of the flanges of different lamps in the same spotlight fixture. Electrical connections to the lamps are in such cases made by attaching individual wires to the individual contacts of the lamps. The contact structures of the lamps accordingly typically are provided with contact screws for the attachment of spade lugs, for example, or tabs for receiving quick disconnect terminals, for example. In these cases, installation or changing of a lamp requires initial connection of wires to the contact elements, followed by positioning of the lamp within an annular seating recess. In all cases, this is a two-handed operation, typically performed by a workman standing on a ladder and requiring considerable care and expenditure of time. 
While plug-in installation of lamps of the type referred to above is feasible, and sometimes practiced, it presents a considerable disadvantage in that, because of the highly disparate nature of the contact structure of the lamps, plug-in sockets have been required to be dedicated to a particular style of lamp, resulting in manufacturing inefficiencies and limitations on the user's ability to change lighting characteristics. 